Starting point for endeavour in which maybe four succeeded, reportedly? (6,3)
I believe the answer is:
Here is my best explanation:
'starting point for endeavour' is the definition.
'maybe four succeeded reportedly?' is the subsidiary indication.
'maybe four' becomes 'square' (four is a square number).
'succeeded' becomes 'won'.
'reportedly?' shows a homophone (sound like).
'won' sounds like 'one'.
'in which' is the link.
(Other definitions for square one that I've seen before include "Back to . . . . . . . . . - at starting point with no progress made" and "back here after failure".)